Many attempts are made for depositing a quinary superconductive oxide in such as, for example, the thallium-barium-calcium-copper-oxide (Tl-Ba-Ca-Cu-O) system or the bismuth-strontium-calcium-copper-oxide (Bi-Sr-Ca-Cu-O) system on an insulating substrate. The sputtering technique is an attractive candidate for the formation of the quinary superconductive oxide, and, for this reason, various targets are proposed for use in the sputtering system.
One of the formation processes starts with preparation of raw powders of thallium oxide, barium carbonate, calcium carbonate and copper oxide and these powders are about 10 microns in average diameter. The raw powders are regulated to a predetermined proportion, and, then, mixed to produce a mixture. The mixture is calcined in a sealed vessel at 600 degrees to 700 degrees in centigrade for a certain time period, and the calcined product is pulverized into a powder. The calcining stage and the pulverizing stage are repeated twice or three times, thereby forming a powder of a superconductive oxide in the thallium-strontium-calcium-copper-oxide system. The powder of the super-conductive oxide is formed into a target by using a hot-pressing technique. Namely, the powder is pressurized at 100 kgf/cm.sup.2 to 200 kgf/cm.sup.2 and subjected to a heat treatment in a vacuum ambient less than 10.sup.-2 torr at 800 degrees to 900 degrees in centigrade for a time period ranging from an hour to four hours. The target thus formed substantially consists of a quinary oxide in the thallium-barium-calcium-copper-oxide system or a composite of the quinary oxide and copper oxide equal to o less than 20 % in volume. The reason why the copper oxide is intentionally included in the target is that the composition of the target is hardly transferred to the deposited film under certain sputtering conditions and, for this reason, the copper oxide is previously supplemented to the powder of the superconductive oxide.
The formation of the target of the quinary superconductive oxide in bismuth-strontium-calcium-copper system traces the similar process sequence to that described above. Namely, a mixture is prepared from raw powders of bismuth oxide, strontium carbonate, calcium carbonate and copper oxide which are about 10 microns in average diameter. A powder of lead oxide may be further mixed into the mixture. The mixture is calcined at 700 degrees and 800 degrees in centigrade for a certain time period, and the calcined product is pulverized into a powder. These are repeated twice or three times, thereby finally producing a powder of superconductive oxide in the bismuth-strontium-calcium-copper-oxide system. With the super-conductive oxide powder, a target is formed by using a hot-pressing technique in the aforementioned conditions. The target thus produced is substantially composed of a superconductive oxide in either bismuth-strontium-calcium-copper or bismuth-strontium-calcium-copper-oxide system. If the excess copper oxide is previously into the mixture so as to adjust the composition of the deposited oxide film, the target contains the copper oxide equal to or less than 20 % by volume.
However, a problem is encountered in the prior art target in production of cracks due to thermal stress. This is because of the fact that the prior art target is small in thermal conductivity, and, for this reason, a large difference in temperature takes place between both sides of the target in the service conditions. The large difference in temperature results in thermal stress which is causative of the cracks. The cracks deteriorates the production yield and, accordingly, increases the production cost of the deposited film.
The prior art target is further small in electric conductivity, so that the prior art target is merely applicable to an expensive r.f. sputtering system. This also increases the production cost of the deposited film.
Another problem inherent in the prior art target is brittleness. The operator is requested to carefully handle the brittle target, however, the operator sometimes makes human errors. This results in destruction of the target, and, for this reason, the production cost of the deposited film further tends to be increased.